Heretofore, polybutadiene rubber has been known as a rubber having thermally and mechanically excellent properties and widely used in various fields. Particularly, since polybutadiene rubber having a high cis-1,4 bond content is excellent in the mechanical properties, the production technique of the polybutadiene rubber having the high cis-1,4 bond content has been actively studied and developed. For example, composite catalyst systems each composed mainly of a transition metal compound of nickel, cobalt, titanium or the like are known as a catalyst for the production of polybutadiene rubber. Among them, some catalysts are industrially used as a polymerization catalyst of butadiene.
As a catalyst capable of producing the polybutadiene having a high cis-1,4 bond content and having an excellent polymerization activity are developed composite catalyst systems each consisting of a rare earth metal compound and an organometal compound of Group I-III. For example, it is reported that when using a catalyst of neodymium compound and methylaluminoxane, the polymerization can be conducted in a high reactivity and a conjugated diene polymer having a narrow molecular weight distribution is obtained. However, the polymer obtained by this method is not sufficiently high in the cis-1,4 bond content and not sufficiently low in the vinyl bond content, so that the properties are still insufficient.
Also, JP-A-2001-48940 discloses that the butadiene-based polymer having a very high cis-1,4 bond content is obtained by polymerizing butadiene in a catalyst system comprising a rare earth metal compound, an organometallic compound of Group I-III and an ionic compound consisting of a non-coordinate anion and a cation. In this case, however, there are problems that a rare earth metal compound used such as Nd(OCOCCl3)3 or the like is low in the polymerization activity. Also, since NMR is utilized as a measurement of the micro-structure in JP-A-2001-48940, the error of the vinyl bond content is particularly large and further the value of the vinyl bond content in the resulting polybutadiene is large, so that the rubber composition containing such a polybutadiene is insufficient in the improvement of the properties as compared with the rubber composition containing the conventional polybutadiene. Furthermore, the polybutadiene obtained by this method tends to widen the molecular weight distribution as the vinyl bond content becomes low, so that there can not be obtained polybutadienes in which the vinyl bond content is sufficiently low and the molecular weight distribution is within a specified range.
Also, JP-A-2002-241420 discloses that the polybutadiene having a very high cis-1,4 bond content is obtained by polymerizing butadiene in a catalyst system comprising a rare earth metal salt consisting of a rare earth metal, a free halogen atom component and a halogen atom containing component and aluminoxane. In this case, however, there are problems that the polymerization activity of bis(trichloroacetic acid)(versatic acid) neodymium salt or the like used is low and the industrial productivity is low. Furthermore, JP-A-2002-241420 does not examine the vinyl bond content of the polymer at all.
In Aida, Hou, Nishiura, Doi, Kurakado, Horiuchi and Wakatsuki, Macromol. Rapid Commun. 2003, vol. 24, pp 179-184 is disclosed that the polybutadiene having a very high cis-1,4 bond content is obtained by polymerizing butadiene in a catalyst system comprising a metallocene complex of a gadolinium compound. However, means for controlling the molecular weight is not disclosed in the above Aida et al report, so that the practicality is low. Also, there is not examined the application of the disclosed polybutadiene to a rubber composition and the application of such a rubber composition to a rubber article, particularly a tire.